One hypothesis I have myself, is that it could be possible for ADHD to be developmentally "caused" by venous/lymphatic insufficiency. In est, the brain notices early on that it can't drain waste amines from the neural cleft through the glymphatic system (because it's underdeveloped), and downregulates their production to compensate.
If that were to be true, then you'd expect to see a high correlation between people who have ADHD, and people who are very sensitive to heat. Blood would be pooling toward their extremeties—though not necessarily to the point of causing edema—and keeping them warm, but also making it easier for the environment to overheat/overcool their blood, somewhat similarly to people with third-degree burns.
It would also suggest that people with ADHD would feel positively about any medication that causes a parasympathetic response, pushing blood toward their core—caffeine being a primary example.
If that were to be true, then you'd expect to see a high correlation between people who have ADHD, and people who are very sensitive to heat. Blood would be pooling toward their extremeties—though not necessarily to the point of causing edema—and keeping them warm, but also making it easier for the environment to overheat/overcool their blood, somewhat similarly to people with third-degree burns.
It would also suggest that people with ADHD would feel positively about any medication that causes a parasympathetic response, pushing blood toward their core—caffeine being a primary example.